Low resistance ohmic contacts on AlGaN/GaN structures using implantation and the "advancing" Al/Ti metallization

The ohmic contact formation of Al/Ti on AlGaN/GaN heterostructure field effect transistors (HFETs) with and without Si implantation was investigated. Direct implantation and implantation through an AlN capping layer were studied. Compared to implantation through AlN, direct implantation is more effective in reducing the contact resistance. An Al(200 Angstrom)/Ti(1500 Angstrom) bilayer structure, called the "advancing'' metallization, was used in this investigation to take advantage of consuming nearly all the top AlGaN layers for easy carrier access to the GaN layer underneath. Combining the direct implantation and the advancing metallization, low contact resistance of the order of 0.25 Omega mm (similar to 5.6 x 10(-6) Omega cm(2)) can be readily obtained on HFET structures with an AlGaN layer about 340 Angstrom thick and with an Al fraction of at least 22%. (C) 1999 American Institute of Physics. [S0003-6951(99)00918-3].